Tiny Mountain Lake, on the southern side of San Francisco’s Presidio Trust, has a big problem.

The historic natural lake has undergone a major restoration but is still teeming with giant, alien, invasive fish species that threaten to thwart efforts to create a proper habitat that won’t scare folks who look in the water. Trapping, gill-netting and hand-catching for three years didn’t work. Neither did electric stunning.

So local, state and federal regulators signed off on a plan to poison the lake next month with a short-lived chemical they hope will kill the giant carp, bass, sturgeon and mosquitofish that dominate the ecosystem. The poison of choice supposedly only harms creatures with gills, i.e. fish.

Workers will dump 47 gallons of Rotenone in a 5% solution into the lake on a single day. Rotenone is a piscicide that suffocates gill breathers. It is mildly hazardous to humans and mammals and has been banned in the United States for use on land, coastal waters and lagoons. It has been banned entirely in Europe. But in this country, it is allowed to be used in freshwater lakes and streams, to the consternation of many wilderness users and scientists.

The chemical will be sprayed over the lake top and pumped into its depths.

Those depths are not very deep. The lake, which scientists think dates back at least 2,000 years, is only 10-12 feet deep now. It used to be three times as deep and 30% bigger. It served as a drinking source for Native Americans and Spanish settlers who later arrived at the Presidio in the 1770s.

But the intervening years have not been kind to Mountain Lake. The U.S. Army took over the Presidio in mid-19th Century, about the same time the lake began to fill with sediment and junk. The army’s golf course in 1897 was said to be a steady source of toxic fertilizer for the lake.

Pollution got nasty during construction of Highway 1 nearby in the 1930s when roadway runoff and its contaminants were directed to the lake. Gasoline runoff was a steady source of lead in the lake. The lake also became a repository for people’s pets. Dumped goldfish became carp. The wrong kind of turtles drove out the native turtles.

Mountain Lake continued to shrink. Rich Shrieve, president of the Friends of Mountain Lake Park, told Bay Nature last year that without human intervention of the healthy variety, it would have been a meadow in a few generations. The Presidio became a national park in 1994 and was included in the Golden Gate National Recreation Area.

Various agencies began to take steps to clean up the lake. Invasive eucalyptus trees were ripped out and replaced with native willows, red alders and wax myrtles. Debris from the trees was pulled out of the lake. Last year, a major dredging project began to remove 15,600 cubic feet of sediment and toxic substances.

After the bad fish are out of the way, the Presidio Trust hopes to bring in “Pacific chorus frogs, western pond turtles, freshwater mussels, Pacific newts and three-spined stickleback,” according to National Geographic.

And then it will all come down to people not mucking it all up because, as Presidio Trust’s director of conservation Terri Thomas told the publication, “Ultimately, urban restoration projects like this depend on the community.”